1. Technical Field
The invention relates to a mask for microlithography. The invention further relates to a scanning projection exposure method for exposing a radiation-sensitive substrate, arranged in the region of an image plane of a projection objective, with at least one image of a pattern of a mask arranged in the region of an object plane of the projection objective, wherein the mask for microlithography is utilized. The invention further relates to a projection exposure system suitable for carrying out the method.
2. Description of the Related Art
Microlithography projection exposure methods and systems are currently used to fabricate semiconductor components and other finely patterned components. A microlithographic exposure process involves using a mask (reticle) that carries or forms a pattern of a structure to be imaged. The pattern is positioned in a projection exposure system between an illumination system and a projection objective in a region of the object plane of the projection objective. Primary radiation is provided by a primary radiation source and transformed by optical components of the illumination system to produce illumination radiation directed at the pattern of the mask in an illuminated field. The radiation modified by the pattern passes through the projection objective, which forms an image of the pattern in the image plane of the projection objective, where a substrate to be exposed is arranged. The substrate normally carries a radiation-sensitive layer (photoresist).
When a microlithographic projection exposure system is used in the manufacture of integrated circuits, the mask (reticle) may contain a circuit pattern corresponding to an individual layer of the integrated circuit. This pattern can be imaged onto an exposure area on a semiconductor wafer which serves as a substrate. The exposure area is sometimes referred to as “target area” or “shot area” or as a “die”. A die in the context of integrated circuit fabrication is a small rectangular block of semiconducting material, on which a given functional circuit is fabricated. A single wafer typically contains a large number of adjacent dies (exposure areas) which are successively exposed to an image of the pattern. The dies are typically arranged in rows and columns.
A typical conventional mask contains one pattern area which includes one complete pattern to be imaged. A narrow light shielding band may be arranged at the edges of the patterned portion to block unwanted radiation from entering the projection objective. A conventional mask may contain two identical complete patterns (see U.S. Pat. No. 5,854,671, FIG. 16A).
In one class of microlithographic projection exposure systems each exposure area is irradiated by exposing the entire pattern of the mask onto an exposure area at once. Such apparatuses are commonly referred to as wafer steppers.
In alternative exposure systems commonly referred to as step-and-scan apparatus or wafer scanner, each exposure area is irradiated progressively in a scanning operation by moving the mask relative to an illumination beam in the object plane of the projection objective, and simultaneously moving the substrate relative to the projection beam in the conjugate image plane of the projection objective in respective scanning directions. The mask is typically held in place by a mask holder, which is movable in a mask scan direction parallel to the object plane of the projection objective. The substrate is typically held by a substrate holder, which is movable parallel to the image plane. Depending on the design of the projection objective the scanning directions of the mask and the substrate may be parallel to each other or anti-parallel to each other, for example. During the scanning operation, the speed of movement of the mask and the speed of movement of the substrate are interrelated via the magnification ratio (absolute value |β|) of the projection objective, which is smaller than 1 for reduction projection objectives.
In a scanning exposure system the illumination system is configured to generate an illumination beam that has a slit shaped cross section in the plane where the pattern is situated. The cross section may be rectangular or arcuate (arc shaped), for example. The area which can be illuminated at a given instant of time is denotes as “illumination slit” in the context of this application.
In a scanning operation the illumination system illuminates a slit shaped portion of the pattern at a given time. A length of the illumination slit in the scanning direction is a fraction of the length of an entire pattern to be illuminated in a single scan. The width of the illumination slit in a cross scan direction orthogonal to the scan direction is larger than the length of the illumination slit and not smaller than the width of the pattern on the mask.
A reticle-masking (REMA) device is usually provided in the illumination system to define the effective length and width dimensions of the illumination slit at a given instant of time. Typically, a reticle-masking device comprises two pairs of movable blades, sometimes denoted as REMA blades. A first pair of blades has edges aligned orthogonal or otherwise traverse to the scanning direction. A distance between these edges in the scanning direction determines the effective length of the illumination slit at a given instant in time. The blades of the first pair can be linearly moved in the scan direction independent from each other. A second pair of blades has edges generally transverse to the edges of the first pair. A distance between these edges in the cross scan direction determines the effective width of the illumination slit.
In exposure systems for microlithography operating with radiation in the deep ultraviolet (DUV) or vacuum ultraviolet (VUV) spectral range a preferred location of the reticle masking device is an intermediate field plane in the illumination system optically conjugate to the plane where the mask is situated. An optical imaging system having a given magnification (often between 1:2 and 6:1) is interposed between the reticle-masking device and the mask and images the edges of the blades sharply onto the mask. Alternatively, the blades of the reticle masking device may be arranged directly in front of the mask. This arrangement is sometimes adopted in exposure systems for microlithography operating with radiation in the extreme ultraviolet (EUV) range of the spectrum. EUV systems use reflective masks.
The blades of the first pair of blades play an important role in controlling the dose of radiation energy incident on each particular point the moving substrate in a scanning operation in the manner of a movable shutter mechanism. Before a scanning cycle starts the blades are effectively closed to prevent radiation from falling onto the mask. As the scanning movement of the mask starts the blades open up to a maximum distance, which is the full illumination slit length for a given scanning operation. At the end of the scanning cycle the blades close again to block any undesired radiation from falling onto the mask. The opening movement is performed by the front blade, i.e. the blade in the forward direction of the scanning movement in synchronism with a front edge of the pattern on the moving mask. The closing movement is performed by the rear blade in synchronism with the rear edge of the pattern of the moving mask (see e.g. U.S. Pat. No. 5,854,671, FIG. 6 and corresponding description). This ensures that each portion of the exposure area is exposed by the same radiation energy dose.